Treatment with any of several VEGF receptor-targeted tyrosine kinase inhibitors (TKIs) results in prolonged disease stability or even regression in the majority of patients with metastatic renal cell carcinoma (RCC). This antitumor effect is generally transient and incomplete, however, due to the rapid development of drug resistance. This project emerged from our earlier work in RCC xenografts on the mechanisms of acquired TKI resistance. We observed that p53 activation was essential for a robust response to sunitinib. We also noted that the expression of p53-dependent genes was transient and down modulated with the onset of TKI resistance. Finally, we demonstrated that the concurrent administration of a drug that blocks HDM2- dependent p53 ubiquitylation and degradation prevented the development of TKI resistance. At least two potential mechanisms for this effect were documented, one of which involved in the induction of the E3 ligase Fbw7 and the degradation of the oncoprotein HIF-2alpha. Another potential mechanism by which HDM2 antagonists prevent TKI resistance is their ability to block the expression of hypoxia-driven chemokines such as SDF-1 (CXCL-12) and to prevent the influx of CD11b+/Gr-1+ myeloid-derived tumor suppressor cells (MDSC). In this project, we propose to carry out a Phase I/Ib clinical trial examining a combination of sunitinib with the HDM2 antagonist CGM097 (Novartis). This trial will have a 20 patient expansion cohort in which the administration of the HDM2 antagonist will be delayed in half of the patients. Tumor biopsies will be performed on these patients (half of which will be receiving sunitinib alone and half the drug combination at the time of biopsy) to assess the effects of treatment on p53 activation and MDSC and Treg trafficking. We will examine the effects of MDSC depletion on treatment outcome in murine RCC models and will determine if targeting the SDF-1 receptor (CXCR4) with the CXCR4 inhibitor AMD11070 prevents MDSC recruitment and the onset of TKI resistance as effectively as HDM2 blockade. Finally, we will assess the effects of HDM2 blockade on the expression of IL-8, FGF, and other factors previously implicated in the development of TKI resistance